This invention relates to target ranging and target activated munitions and in particular to an acoustic system that predicts the closest point of approach to a munition and the time of arrival thereat of a moving target and generates a munition fire signal in response thereto.
There currently exists a military requirement for a system that is capable of automatically detecting and classifying military targets and developing firing signals for special munitions at the appropriate target position and time. It is a specific requirement of such a system to detect ground-based targets through their seismic, acoustic and/or other characteristic emanations and to produce an electrical firing command when the target is at its closest point of approach and within the predetermined effective range of the munition. Physical constraints on the requirement dictate that the sensor must be passive and act independently from a single point. Further the concept of the system must be such that the sensor system can ultimately be reduced in size, power consumption and cost to be placed within the munition.
These constraints essentially eliminate the use of conventional radar devices because they are not passive and also because of their size, cost and complexity.
One prior art approach to providing the type of system required comprehends the concept of single site ranging by cross-correlation of seismic and acoustic signals. Such a concept depends upon or infers a knowledge of the velocity of propagation over two paths. The speed of sound in air is reasonably constant being mildly dependent on air temperature. If accuracy requirements dictate the need, air temperature could be sensed and the necessary corrections performed. However, obtaining a knowledge of the seismic velocity over short ranges and in the top soil and weathered layer is extremely difficult. The details of runway construction and other man-made changes in the natural terrain as well as naturally occurring rapid changes in soil conditions from point to point on the surface make the advance prediction of seismic velocity impractical. Systems of this type therefore have never been effectively developed.
Another approach utilizing the concept of single site ranging by an acoustic single sensor method has been developed and is described in our co-pending patent application Ser. No. 286,820, entitled Acoustic Amplitude Threshold Target Ranging System, filed on even date herewith. Although this system generally satisfies the above stated requirements it lacks the precision necessary for some applications.
Accordingly, there currently exists the need for a target ranging system for munition activation that is passive and acts independently from a single point and that is small, inexpensive and low in power consumption. It is also required that such a system have a precise target location predictive capability. The present invention is directed toward satisfying that need.